In a typical pressure regulator, an actuator, usually a diaphragm, is mechanically connected to a flow control valve, said actuator being designed to assume an equilibrium position proportional to the difference in pressure across a pressure sensing area thereof whereby as the regulated pressure increases, a force is created on the actuator which counteracts a spring load thereon and, upon compression of the spring, the valve will move toward passage closing position. As the regulated pressure decreases, the actuator will urge the valve in a direction to increase the rate of flow of fluid. Such typical regulator tends to be unstable, that is, as the pressure which is desired to be regulated increases, the valve opens excessively with resulting increase of pressure above desired value whereby the valve closes too far with resulting decrease of pressure below desired value. In many applications the resulting oscillation is not acceptable. A basic cause of instability in the typical regulator is that when the valve is nearly closed, say 0.0001 inch from its seat a very small change in valve position, say 0.001 inch, will cause a very large proportional change in flow area and, therefore, a very large change in flow rate. This large change in flow rate typically causes a rapid change of pressure so that the pressure goes too high before the actuator has time to react. The actuator, then belatedly sensing the too high pressure overcompensates by closing the valve too far causing an undershoot.
In addition, with only a single valve seat numerous cycles are incurred in response to small flow demands which cause a very short life. Additionally, a single valve operating close to the seat is sensitive to contamination damage and accelerated erosion and shortened life.
As known, flow forces and particularly in the case of two-phase flows also tend to cause oscillating forces in the moving parts of the regulator and because the moving parts constitute a spring mass system it has a resonant frequency so that oscillating forces tend to cause amplification of oscillating movement. A basic way to control resonant oscillation is to introduce the proper amount of friction, but the friction has a rather narrow tolerance because too little friction produces insufficient damping and too much causes excessive regulation error. Where such friction is provided, it is apt to change during the service life or operating conditions of the regulator and is difficult to accurately maintain in the manufacture of the regulator. In the case of a cryogenic application, the valve actuator must be made of materials that will withstand low temperatures, e.g. down to -420.degree. F. and this results in shortened life.